1. Technical Field
The present invention relates to load cells or force transducers having multiple shear areas within the body for strain measurement, and more particularly to such load cells that have full redundant high precision axial sensitivity with insensitivity to extraneous forces and couples.
2. Background Art
Force transducers used to measure axial loads are found in nearly all actively controlled mechanical systems. A particularly stringent application is that of measuring an applied force in order to control the shape of a deformable optic. Such applications require high accuracy, long-term stability, and full redundancy of the actual sensing elements since the force actuator which employs the force transducer will be in aerospace devices. Another key requirement is the ability to sense axial forces only while remaining insensitive to other extraneous applied forces. A variety of methods both mechanical and electrical have been devised to compensate for limitations in these aspects of load cell performance.
A simple shear mode axial force transducer, U.S. Pat. No. 3,513,431 to Kovacs, uses multiple gauges placed circumferentially about the transducer body to partially compensate for nonaxial loading. U.S. Pat. No. 4,162,628 to Oetjen et al, has increased stiffness and employs larger shear panels arranged symmetrically around the transducer body. Both designs are multiple gauges in each bridge leg to compensate for side loading. U.S. Pat. No. 3,969,935 to Shoberg, employs a unitary transducer body using a stress concentration to increase axial strains while reducing the effects of off-axis forces. A combination of this approach and circumferentially locating gauges is shown in U.S. Pat. No. 4,423,793 to Caris.
None of the above designs fully compensates for moments applied about the measuring axis. U.S. Pat. No. 3,958,456 to Jacobson employs multiple gauges on multiple surfaces to electrically subtract effects of applied moments and forces. Gauges can also be configured such that forces applied to all three axes can be determined such as in U.S. Pat. No. 3,771,359 to Shoberg. These are complicated, expensive and difficult to balance and calibrate. Another method of reducing sensitivity to extraneous force is to isolate the measuring beam from all but the desired axial load. This was illustrated in U.S. Pat. Nos. 4,009,608 and 4,546,838 to Ormond by using parallel beam flexures to isolate the shear panel from extraneous loads. The use of special strain gauge design and critical placement to compensate for offset loads is disclosed in U.S. Pat. No. 4,657,097 to Griffen. Moment compensation for a single axis shear sensor is illustrated in British Patent No. 1,456,40 to Hindle.